The present invention relates to a cold cathode field emission device and a cold cathode field emission display into which the cold cathode field emission device is incorporated.
Various flat type, or flat panel type displays are being studied as an image display which is to replace currently main-stream cathode ray tubes (CRT). The flat type displays include a liquid crystal display (LCD), an electroluminescence display (ELD) and a plasma display panel (PDP). Further, there is also proposed a cold cathode field emission display capable of emitting electrons into a vacuum from a solid without relying on thermal excitation, and it attracts attention from the viewpoint of a brightness on a screen and a low power consumption.
The cold cathode field emission display (to be sometimes simply referred to as "display" hereinafter) generally has a configuration in which a cathode panel and an anode panel are arranged so as to face each other through a vacuum layer. The cathode panel has electron emission portions corresponding to two-dimensionally arranged pixels having a gridiron pattern. The anode panel has a fluorescence layer which emits light by its excitation due to a collision with electrons emitted from the electron emission portions. On individual pixels on the cathode panel, generally, a plurality of electron emission portions are formed, and gate electrodes are also formed for emitting electrons from the electron emission portions. An element constituted of the above electron emission portion and the above gate electrode will be referred to as a cold cathode field emission device or a field emission device hereinafter.
In the above display, for attaining a large emission electron current with a low driving voltage, each electron emission portion is required to have a sharply pointed form, it is required to scale down electron emission portions in a block corresponding to one pixel for increasing the density of the electron emission portions, and it is required to decrease a distance between the top end portion of each electron emission portion and each gate electrode. Cold cathode field emission devices having various configurations have been so far proposed for complying with the above requirements.
As a typical example of conventional field emission devices, there is known a so-called Spindt-type field emission device having electron emission portions composed of an electrically conductive material having a conical form. On the cathode panel side of a display into which the Spindt-type field emission devices are incorporated, a cathode electrode, an insulating layer and a gate electrode are consecutively formed on a supporting substrate. Many fine opening portions having a diameter of approximately 1 .mu.m are formed in a two-dimensional matrix form so as to penetrate through the gate electrode and the insulating layer, and the electron emission portions are formed on the cathode electrode exposed in bottoms of the opening portions. When a voltage is applied to the gate electrode constituting an edge of the opening portion, electrons are emitted from the top end portion of the electron emission portion depending upon the intensity of an electric field generated by the voltage application. Emitted electrons are drawn out of the opening portion and collide with the fluorescence layer on the anode panel side to excite the fluorescence layer and to allow the fluorescence layer to emit light, so that the electrons serve to form an intended image. The conical electron emission portion composed of an electrically conductive material is formed, in a self-aligning manner, by decreasing the amount of depositing particles of the electrically conductive material which can strike into the opening portion, with the passage of time by utilizing the shielding effect of an overhung deposit of the electrically conductive material deposited around the edge of the opening portion during the vertical deposition of the electrically conductive material.
The electron emission characteristic of the Spindt-type field emission device is largely dependent upon the distance from the edge of the gate electrode constituting the edge of the opening portion to the top end portion of the electron emission portion. Actually, however, it is difficult to form the electron emission portions having a uniform form and uniform dimensions in the entirety of the supporting substrate having a large area, and some in-plane deviation and a deviation among lots are inevitable. The deviations cause image displaying characteristics of a display, for example, a brightness of images to vary.
For overcoming the above defects of the Spindt-type field emission device, a so-called edge-type field emission device has been proposed. In one example of the edge-type field emission device, the conical electron emission portions in the Spindt-type field emission device are replaced with projections formed by consecutively forming, on an insulating substrate as a supporting substrate, a first insulating layer, an electron emission layer, a second insulating layer and a gate electrode to form a laminate, forming an opening portion in the laminate, and projecting an edge (end portion or the projection) of the electron emission layer by some method, which edge is exposed on a wall surface of the opening portion.
As a method of projecting the edges of the electron emission layer from the wall surfaces of the opening portions, generally, there is employed a method in which the above laminate is processed by combining anisotropic etching and isotropic etching. That is, the gate electrode is etched under an anisotropic condition, the second insulating layer immediately below the gate electrode is etched under an isotropic condition, the electron emission layer immediately below the second insulating layer is etched under an anisotropic condition, and the first insulating layer immediately below the electron emission layer is etched under an isotropic condition, whereby the wall surfaces of the first insulating layer and the second insulating layer are "withdrawn" more deeply than the edge of the gate electrode and the edge of the electron emission layer. In the above configuration, the distance from the edge of the gate electrode to the edge of the electron emission portion is mainly dependent upon the thickness of the second insulating layer, and it is far easier to control the above distance than to control the distance in the Spindt-type field emission device. Therefore, uniform electron emission characteristics of the electron emission portions can be accomplished even on the supporting substrate having a large area, and a uniform brightness of an image on a display can be also accomplished.
U.S. Pat. No. 5,214,347 discloses a structure in which not only a gate electrode is formed on the upper side of the electron emission layer but also a gate electrode is formed on the lower side of the electron emission layer so that a more intense electric field can be applied to the electron emission layer. That is, as shown in FIG. 18, a conductive layer 101, a first insulating layer 102, a lower gate electrode 103, a second insulating layer 104, an electron emission layer 105, a third insulating layer 106 and an upper gate electrode 107 are consecutively formed on a supporting substrate 100 to form a laminate, and an opening portion 108 is formed which penetrates through all the layers excluding the conductive layer 101 and has the conductive layer 101 exposed on a bottom thereof. Predetermined voltages are applied to the lower gate electrode 103, the electron emission layer 105 and the upper gate electrode 107 to generate an electric field, and due to the electric field, electrons e are emitted from the edge of the electron emission layer 105 projected on the wall surface of the opening portion 108. The emitted electrons are introduced out of the opening portion 108. The top of the edge of the electron emission layer 105 has its radius of curvature decreased by decreasing the thickness of the electron emission layer by isotropic etching, whereby the electron emission density is increased.
A conductive layer 109 disposed so as to face the upper gate electrode 107, the electron emission layer 105 and the lower gate electrode 103 constitutes an electrode for attracting electrons emitted from the electron emission layer 105. The conductive layer 101 exposed on the bottom of the opening portion 108 is provided for surface protection, potential stabilization and prevention of dielectric breakdown and a noise.
In the edge-type field emission device disclosed in U.S. Pat. No. 5,214,347, the electron emission layer 105 which constitutes electron emission portions can be formed nearly in the form of a flat plate or layer and unlike the above Spindt-type field emission device, it is not required to sharpen the electron emission portions three-dimensionally, so that the edge-type filed emission device can be easily produced as compared with the Spindt-type field emission device.
In the above edge-type field emission device, further, the distance from the edge of the gate electrode 103 or 107 to the edge of the electron emission layer 105 can be mostly determined on the basis of the thickness of the insulating layer 104 or 106. It is therefore far easier to control the above distances than it is in the Spindt-type field emission device. In this sense, the defects of the Spindt-type field emission device can be overcome to a considerable extent. The uniform electron emission characteristics of the electron emission portions can be therefore easily accomplished even on the supporting substrate having a large area, and a uniform brightness of an image on a display can be also accomplished.
The problem with the field emission device is that the electron emission characteristics of the electron emission portions vary. When the potential difference .DELTA.V between a voltage applied to the gate electrode and a voltage applied to the electron emission layer comes to be greater than a certain threshold voltage, electrons are begun to be emitted from the edge of the electron emission layer. With an increase in the voltage applied to the gate electrode (i.e., an increase in the potential difference .DELTA.V), an emission electron current I generated by the emission of electrons from the edge of the electron emission layer sharply increases. Further, when the emission electron current I exceeds a limit value I.sub.MAX, the edge portion of the electron emission layer is destroyed.
The electron emission portions are formed on a cathode panel in a unit of as many as several hundred thousand to several hundred million under the same process, and even when field emission devices appear to be uniform through an electron microscope, the threshold voltages of the field emission devices vary. In such a state, field emission devices having characteristics D.sub.1 and D.sub.2 shown in V-I curves of FIG. 19B are destroyed by an overcurrent. Field emission devices having characteristics D.sub.3 and D.sub.4 emit electrons. However, field emission devices having characteristics D.sub.5 and D.sub.6 do not begin to emit electrons from edges of the electron emission layer since the potential difference is lower than the threshold voltage. In FIGS. 19A and 19B, the axis of abscissas indicates potential differences .DELTA.V, and the axis of ordinates indicates emission electron currents I. When the threshold voltages of the field emission devices vary, some field emission devices emit electrons from the edges of the electron emission layer, and some do not, even if the potential difference is constant .DELTA.V. Further, there are actually a potential variation in the range of several volts between adjacent lines, which consequently causes variation in brightness between the lines. It is assumed that the above potential variation and the threshold potential variation are caused by microscopical differences in surface states of the electron emission portions, while it is not necessarily clear what causes the above phenomena, and current production techniques inevitably involve them. There is also another problem that the electron emission characteristic of the electron emission portion comes to be non-uniform with the elapse of time. As a result, there is caused a problem that it is difficult to display clear images with a conventional field emission device or that images cannot be stably displayed.